I would say that the Commercial customer might need to ensure the two Romanian guys are legitimately self employed, legally insured, have health and safety documentation in place or he could be very vulnerable if:
a) the previous window cleaner feels the need to check whether the new guys comply with criminal legislation.
b) the new guys find a better job and leave him in the lurch
c) a claim is made due to an injury to the new guys and the Commercial customer hasn't taken a duty of care towards them.
Of course all these things may be in place but who knows?
As a back stop mightn't your employee go in at a cheaper rate himself?